Method and system for multiple hosts anycast routing

ABSTRACT

In case the first indicator specifies a number of anycast members greater than the second indicator, the number of nearest anycast members indicated by the second indicator (in the following: second number of anycast members) are skipped and not contacted, while a number of anycast members is determined for data communications with the sender up to a number obtained by a comparison of the number of anycast members specified by the first indicator (in the following: first number) and the second number. In particular, the number of nearest anycast members to be determined corresponds to the differen e between the first number and the second number. In case the first number corresponds to the second number, no further anycast members have to be determined. The same applies for the case wherein the first number is smaller than the second number.

TECHNICAL FIELD

The present invention relates to a method and a system for establishingdata communications between a sender and receivers, the number thereofbeing defined by the sender, wherein the receivers are not individuallyaddressed by the sender, but selected from a group of receivers beingaddressed from the sender by means of a common address.

BACKGROUND OF THE INVENTION

An anycast network is a network providing a service which allows asender to access the nearest of a group of receivers having a commonaddress. The common address, in particular, the same anycast address forthe group of receivers enables a sender to identify a plurality ofreceivers by providing only one address, namely an anycast address,instead of individually addressing each receiver. In contrast to amulticast system, wherein data communications are performed between asender and all receivers sharing a common address, i.e. the samemulticast address, data communications in an anycast network areperformed between a sender and a particular receiver of a group ofreceivers sharing the same anycast address. In the context, datacommunications include any kind of data, such as alphanumerical,graphic, multimedia, audio, video, voice data, information, signals etc.which can be exchanged between systems, devices, network components etc.(e.g. computers, end user devices, hosts, servers, routers, modes).

In particular, data communications in an anycast network are performedbetween a sender and the nearest receiver of the group of receiverssharing the same anycast address. In this respect, the nearest receiverin relation to a sender is defined according to a measure of a distanceemployed by a routing protocol or routing system used for datacommunications in a respective network.

Since the routing protocol or routing system of a network whichidentifies the nearest receiver for accesses by a sender, the senderdoes not need to care about how to select the closest destination, i.e.the nearest receiver.

Usually, receivers in an anycast group are so-called replicas being ableto support a same network service for requesting sender. Examples forsuch replicas are mirrored web servers. For accessing a desired networkservice, among a group of receivers sharing the same anycast address,i.e. anycast receivers, each thereof providing the desired networkservice, the nearest receiver is determined and respective datacommunications are performed between the nearest receiver and therequesting sender. Thus, accessing the nearest receiver enhances thenetwork performance perceived by the requesting sender, saves networkcapacities such as network's bandwidth, and provides the desired networkservice.

In FIG. 1, the basic principle of an anycast network is illustrated.Both, members M1 and M2 are members of the same anycast group and sharethe same respective anycast address. In line with the measure of adistance employed for routing of data in the network, the distancesbetween requesting senders S1 and S2 and all members of the anycastgroup are computed. Since the distance 2 calculated for the sender S1 inrelation to the anycast group members is the smallest distance, datacommunications will be performed between the sender S1 and the member M1acting as a receiver for the sender S1. In this manner, it is possiblefor example to direct Internet Protocol (IP) datagrams from the senderS1 towards the member M1. Accordingly, data communications areestablished between the sender S2 and the member M2 being a nearestreceiver with respect to the sender S2, for example to direct IPdatagrams from the sender S2 towards the member M2.

Anycast Addresses

According to the Internet Protocol version 6 (IPv6), special anycastaddresses have been specified in addition to unicast addresses forindividually addressing single receivers and multicast addresses foraddressing all receivers sharing the same multicast address. Anycastaddresses are allocated from the address space employed for unicastaddresses, using any of the formats defined for unicast addresses. As aresult, anycast addresses cannot be syntactically distinguished fromunicast addresses. An anycast address identifies a set of interfaceswhich typically belong to different nodes of a network. Data, e.g. adata packet, communicated from a sender to an anycast address isdelivered to one of the interfaces identified by the respective anycastaddress, in particular, to the nearest interface according to themeasure of a distance employed for the network routing protocol orsystem.

In case, a unicast address is assigned to more than one interface, thusturning the unicast address into a respective anycast address, the nodesof the network to which the address is assigned must be explicitlyconfigured to interpret the address as an anycast address. According tothe IPv6, an anycast address must not be assigned to a host, but may beassigned to an IPv6 router, only.

Usually, for any assigned anycast address, an address prefix P isutilized which identifies a topological region in which all interfacesbelonging to an anycast address reside. In the region identified by theaddress prefix P, each member of the respective anycast group isadvertised as separate entry in a network routing protocol or system.Outside the region identified by the address prefix P, the anycastaddress may be aggregated into the routing advertisement for the addressprefix P.

Anycast Host Registration

In order to join an anycast group, a host instructs its first hoprouter, i.e. the closest router in relation to the host for datacommunications via a network, to advertise the anycast address of theanycast group on its behalf. This can be achieved by adding a newmessage type to either the Internet Group Management Protocol (IGMP) orthe Neighbor Discovery Protocol. Then, the first hop router advertisesthe anycast address according to an anycast routing protocol employedfor the domain including the host.

Anycast Routing

Each anycast group is confined to a particular topological region withwhich it shares a common address prefix. Within the topological regionidentified by the shared address prefix, each member of the anycastgroup is advertised as a separate entry in the employed unicast routingprotocol or system. The principle of anycast routing within atopological region identified by an address prefix shared by an anycastgroup is illustrated in FIG. 2.

As can be derived from the table in FIG. 2, there are multiple paths tothe anycast address. In case, multiple paths exist to a destinationaddress prefix, a routing table look up of the router will returnmultiple next hops. The selection of the next hop router to be used fordata communications, e.g. a particular data packet, depends on theimplemented routing protocol or system. Further, the path selected fordata communications can also be effected by the type of datacommunications to be performed. For example in case of the InternetProtocol standard, the Type Of Service (TOS) field in the IP header of adata packet can be used to define a data communications path. Thus, aTOS designation of a data packet would help the router to chose anappropriate communication path for the given data packet.

The selection of an appropriate path is accomplished on the basis on adetermination of the distances to the destination defined by the addressprefix according to a measure of distance of the respective routingprotocol or system. In case of the Internet Protocol standard, forexample, the OSPF knows the distance related to the corresponding matrixas identified in the TOS field of a data packet to the destination, i.e.the hop count. As a result, for an anycast routing it is possible toselect the nearest next hop on the basis of the employed matrix. In thisrespect, it is not necessary to analyze the whole IP address of a givendata packet. In FIG. 2, this in indicated by the destination address Mxwherein only the prefix “M” is analyzed.

For an anycast routing outside the topological region identified by theshared address prefix, the anycast address may be aggregerated into therouting advertisement for the shared address prefix. This principle isillustrated in FIG. 3.

The destination address Ax in FIG. 3 denotes that only the sharedaddress prefix is utilized in order to determine that datacommunications are to be routed to the corresponding domain. Since theanycast address for the anycast group shares the address prefix withnetwork domain A, network domain B cannot aggregerate the anycastaddress in its address prefix. Therefore, network domain B shouldadvertise the anycast address as a separate entry covering both anycastmembers A4 and A5. This is indicated in FIG. 3 by the destinationaddress Ax being based on the address prefix for domain A anddestination address Ax′ containing the complete anycast address. Therouting for data communications within the network domains A and B isperformed as described before with reference to FIG. 2.

Strict and Loose Routing

According to the Internet Protocol version 4 (IPv4) and the InternetProtocol version 6 (IPv6), the following options for routing datacommunications from a source (e.g. sender, requesting system) to adestination (e.g. receiver, accessed system) have been defined as partof the IP data packet header:

-   Strict Routing

This option defines a complete data communications path from a source toa destination by means of a sequence of IP addresses. Data to becommunicated between the source and the destination is required toexactly follow the defined path.

-   Loose Routing

This option specifies a number of routers and an order thereof. Data tobe communicated from a source to a destination is required to traversethe specified routers in the specified order, but is allowed to becommunicated via other routers on its way from the source to thedestination.

Problems

Existing solutions for networks being capable of anycast routing arerestricted to the selection of a single nearest receiver for datacommunications with an accessing sender. In case a sender intends toaccess more than one receiver, existing anycast networks do no providesuch a service. A situation for which accesses to more than one receiverare desired is, for example, a network user wanting to access a numberof nearest network servers in order to negotiate for the best networkservice conditions. A further example is a network user wishing tocontact a number of nearest restaurants by accessing the network serversthereof in order to check e.g. meals, prices, available seats, etc. orwishing to access a number of nearest emergency service providers inorder to ensure that at least one is able to assist (note for the latterexamples, sufficient information for the accessing user is only providedin case there is a geographical relationship between the restaurants andthe emergency service providers and the corresponding servers).

In conventional networks, accesses to more than one receiver can beaccomplished by accessing a number of receivers by individuallyaddressing the same on the basis of a unicast routing or by accessing agroup of receivers by commonly addressing the same on the basis of ashared multicasting address. Such a multiple unicast routing requiresthat an accessing user must know each individual network address of thedesired receivers and individually perform accesses thereto. This is atime consuming procedure which cannot lead to the desired informationfor a user in a case, for example, he/she is not provided the respectivenetwork addresses and/or information concerning distances to theaccessed servers. With respect to the above given example concerning anegotiation for the best network service conditions by accessing anumber of nearest servers, such a multiple unicast routing is also notappropriate since no information indicative of distances to receivers isprovided and/or available. Accesses on the basis of a multicast routingcan result in a number of contacted receivers being too high in view ofthe demands and desired information of a requesting sender/user.Further, with multicast routing it is not possible to access a desirednumber of receivers and, in particular, a desired number of nearestreceivers, since all receivers being a member of the respectivemulticasting group are addressed together.

Furthermore, a second attempt, e.g. by a multiple unicast routing, toaccess/contact receivers, the number thereof being higher compared tothe number of receivers specified in a preceding, first attempt, wouldunnecessarily return the results from the first attempt again. Thisrepetition of data communications with respect to the result of thefirst attempt unnecessarily utilizes network resources in case therequesting network user is not interested in being provided the firstattempt results again.

OBJECT OF THE INVENTION

The object of the present invention is to provide for a method and asystem allowing for accesses of a sender to a specified number ofnearest receivers and/or data communications between a sender and aspecified number of nearest receivers.

The approach underlying the present invention to obtain the above objectis to extend principles employed in existing anycast networks foraccessing a single nearest client in order to allow for a multipleanycast routing to a specified number of nearest receivers. Inprinciple, this is achieved by associating data communications from asender directed to a network by means of an anycast routing to datawhich indicate the number of nearest receivers. In particular, such datawhich will be referred to as a first indicator in the following,specifies the number of nearest receivers being anycast members of ananycast group identified by an anycast address given by the sender(left) to be contacted by and/or allocated for the sender for datacommunications, such as requests, accesses, data transmissions, etc.

According to the present invention, a method for multiple hosts anycastrouting in a network is provided wherein sender data is communicatedfrom a sender to an anycast group including anycast members. Forcommunicating the sender data to the anycast group, the sender dataincludes an anycast address for addressing the anycast group. In orderto specify a number of the anycast members for data communications withthe sender, the sender data further includes a first indicator beingindicative of the desired, first number of anycast members. On the basisof the sender data, a number of nearest anycast members is determined ashosts with respect to the sender for data communications. In particular,the number of nearest anycast members being determined is defined by thefirst indicator.

This solution provides for data communications between a sender and anumber of receivers without addressing the same separately, butaddressing a plurality of possible receivers by a single address beingcommon for all receivers and selecting a desired number therefrom bymeans of a data being simply amended to the single common address. Afurther benefit is that it can be employed for any network, even fornetworks not providing known anycast routing services.

After having performed the above described method it may be possiblethat a further multiple host anycast routing should be performed whereinthe number of anycast members to be determined is varied compared to thenumber of anycast members previously specified. In order to avoid thatanycast members already determined in the previous multiple host anycastrouting are contacted again a second indicator can be used. The secondindicator specifies the number of anycast members previously determinedin the preceding multiple host anycast routing. Then, the number ofnearest anycast members can be determined on the basis of the first andthe second indicator.

In case the first indicator specifies a number of anycast membersgreater than the second indicator, the number of nearest anycast membersindicated by the second indicator (in the following: second number ofanycast members) are skipped and not contacted, while a number ofanycast members is determined for data communications with the sender upto a number obtained by a comparison of the number of anycast membersspecified by the first indicator (in the following: first number) andthe second number. In particular, the number of nearest anycast membersto be determined corresponds to the difference between the first numberand the second number. In case the first number corresponds to thesecond number, no further anycast members have to be determined. Thesame applies for the case wherein the first number is smaller than thesecond number.

In order to provide for a method according to the invention for multiplehosts anycast routing to be employed in a network providing a knownanycast routing, a first indicator specifying a first number of anycastmembers to be contacted/allocated for data communications is associatedto an anycast address. By means of the anycast address, an anycast grouphaving anycast members is identified in the network. To establish thedesired data communications, at least the anycast address and the firstindicator is communicated in the network to the anycast group. Theanycast group is identified by the anycast address, while the number ofanycast members to be contacted/allocated for data communications isselected according to the first indicator. In particular, the firstindicator defines the number of nearest anycast members and according tothe first indicator, the specified number of nearest anycast members isdetermined as hosts for data communications in the network.

Comparable to the embodiment of the above described method for multiplehosts anycast routing in a network, this method for multiple hostsanycast routing in an anycast capable network can employ a secondindicator specifying a number of anycast members (in the following:second number) already determined for data communications in thenetwork, e.g. on the basis of a previously performed multiple hostsanycast routing in a (anycast capable) network.

For providing the first and/or the second indicator, it is possible toinclude the indicator(s) in the anycast address or to extend the same bythe first and/or the second indicator.

Preferably, the anycast address is included an anycast address prefixwhich identifies a topological region in the network, the topologicalregion confining the anycast group. Here it is possible, that theanycast address prefix includes the first and/or the second indicator oris extended by the same.

In a case of a network being operated according to an Internet Protocolstandard (e.g. IPv4 or IPv6) the anycast address and the indicator canbe communicated by a transmission of an Internet Protocol data packetheader which includes at least the anycast address.

For communicating the first and/or the second indicator, the InternetProtocol data packet header can be included the first and/or the secondindicator as a part of or an extension for the anycast address. Withrespect to the options mechanisms defined for the Internet Protocols,the first and/or the second indicator can be provided by extending theInternet Protocol data packet header by an options field and, inparticular, a so-called multiple anycasting options field including thefirst and/or the second indicator.

It is preferred, that the property “nearest” of anycast members isdetermined according to a measure of distance of the respective routingprotocol employed for the network. In particular, “nearest” may beunderstood as the shortest distance between a sender and anycastmembers. Additionally or as an option, “nearest” may be specifiedaccording to a distance metric requested by the sender. Usually, thedistance metric requested by the sender, e.g. in the TOS field of an IPheader, indicates a smaller distance metrics or nearest distance,respectively.

In case the network comprises domains at least some of which includingat least of the anycast members, the anycast address and the firstand/or the second indicator can be communicated via network routers toand/or between domains having anycast members.

Since it is assumed that a nearest network domain will also includeanycast members being nearer compared to anycast members of more distantdomains, it is preferred that the ordering of routers for network domaincommunications is set up such that the anycast address and the firstand/or the second indicator are communicated to nearest network domains.The nearest network domains may specified according to a measure ofdistance of a routing protocol for the network and/or according to a(e.g. smallest) distance metric requested by the sender, e.g. in the TOSfield.

In order to provide for a fast multiple anycasting, the number ofanycast members can be contacted sequentially, in particular for a smallnumber specified by the first and/or the second indicator.

Within a network domain, multiple anycast routing and, in particular, acommunication of the anycast address and the first and/or the secondindicator can be accomplished on the basis of data identifying each ofthe anycast members. Such an identifying data can be, for example,provided by means of separate entries for anycast members in a routingtable according to the employed routing protocol.

With respect to the anycast routing performed for a network domain, thefirst indicator can be updated by decreasing the same according to anumber of anycast members comprised by a network domain being identifiedby means of the anycast address and receiving the first indicator.

Further, the anycast routing can be enhanced by multiplying the anycastaddress and the first and/or the second indicator and communicating themultiplied anycast addresses and indicators to network domains and/oranycast members. Here, it is preferred to communicate the multiplieddata simultaneously to the further network domain and/or anycastmembers.

In order to precisely select the specified number of anycast members,while determining the number of anycast members, the indicator ismodified according to a number of already determined/contacted/allocatednearest anycast members such that the modified indicator beingcommunicated in the network specifies a number of anycast members stillto be determined as receivers. This embodiment avoids a utilization offurther data beside the indicator to provide information how manyreceivers are already obtained and how many are still to be selected.

According to an Internet Protocol standard, the communication in thenetwork can be performed by using the strict routing mechanism or theloose routing mechanism.

Preferably, the first and the second indicator are communicated togetherand, advantageously in the same manner, e.g. both indicators beingincluded in the anycast address or extending the same.

Further, the present invention provides a system for multiple hostsanycast routing and an anycast routing capable network which arepreferred to be operated according to the method according to theinvention as set forth above.

Moreover, the present invention provides a computer program productbeing adapted to carry out the method steps according to the inventionand embodiments thereof as set forth above.

BRIEF DESCRIPTION OF THE FIGURES

In the following description of preferred embodiments of the presentinvention it is referred to the accompanying figures wherein:

FIG. 1 illustrates the basic principle of anycast routing in a networkaccording to the state of the art,

FIG. 2 illustrates an anycast routing within a topological regionidentified by a common address prefix according to the state of the art,

FIG. 3 illustrates an anycast routing outside a topological regionidentified by a common address prefix according to the state of the art,

FIG. 4 illustrates a data structure for an anycast address according tothe present invention, and

FIG. 5 illustrates a multiple anycast routing according to the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In order to promote the understanding of the present invention preferredembodiments are described with respect to an network according to theInternet Protocol standard. In such a network, data communications areperformed by transmitting IP data packets, short packets, each includingan IP packet header, short header, between hosts, i.e. sending andreceiving systems within the network. Each host is associated to anetwork domain defining a topological network region. Datacommunications between network domains are performed via exteriorgateway routers, whereas data communications within a network domain areperformed via interior gateway routers. Exterior gateway routersdirectly receiving packets from a network domain are called firstexterior gateway routers and interior gateway routers directly receivingpackets from an exterior gateway router are called border routers. Hopsdesignate network components via which packets are communicated. Inparticular, hops include hosts and routers. For data communications inthe network, i.e. routing of packets from a source to a destination,different routing protocols are used for transmission within a domain(intra-domain routing) and between domains (inter-domain routing). Forexample, an intra-domain routing can be based on interior gatewayrouting protocols, such as OSPF, while inter-domain routing can employexterior gateway routing protocols such as BGP. According to the routingprotocols, a routing table is defined which specifies a communicationpath for packets from a source to a destination and, in particular, hopsvia which packets are to be transmitted. As explained above, both, looserouting and strict routing are possible, whereas for a multiple hostsanycasting most probably loose routing will be applied.

IP packet headers include information specifying a destination in thenetwork to which packets are to be communicated from a source by meansof destination addresses. In case of unicast routing a destinationcorresponds to a receiving host, short receiver, whereas in case ofmulticast routing, conventional anycast routing, and, specifically,multiple host anycast routing a destination comprises a group of hostseach thereof, only one thereof, and a specified number thereof acting asreceiver(s) with respect to a sending host, short sender.

In case of a multiple hosts anycast routing, short multiple anycasting,the number of hosts, i.e. members of an anycast group, to be contactedis specified by extending a packet and, in particular, its header by anindicator. The indicator providing information of the number of hosts tobe contacted/allocated as originally specified by the sender and thenumber of hosts which still have to be contacted/allocated as receiversindicates the number of hosts that (still) have to be informed by thecorresponding branch utilizing the following embodiments.

Indicator as Part of the IP Destination Address Field in the IP Header

Since anycast routing is mostly based on resolving the above describedaddress prefix, data (e.g. some bits) of an IP destination address and,in particular, an address prefix is used for the indication of asender-specified number of nearest hosts being anycast members of ananycast group indicated by the address prefix.

In the exemplary illustration of FIG. 4, the IP packet header includesan anycast address prefix and an indicator specifying the number ofnearest anycast members (left) to be contacted/allocated. Initially, theindicator specifies the number of nearest anycast members as desired bya sender, while communicating the packet within the network theindicator is decreased corresponding with the number of already selectedreceivers (already allocated anycast members).

For the examples given above wherein more than one host is to becontacted as receiver, it is assumed that the sender specified number ofdesired nearest anycast members is rather low, e.g. less than 10. Insuch applications of multiple anycasting, it might be sufficient to, forexample, use 4 bits of an IP packet header for the indicator to allowfor a maximum of 2⁴(16) nearest anycast members to be contacted. Inapplications wherein a specification of a higher number of nearestanycast members should be provided, the amount of data (e.g. number ofbits) of an IP packet header is to be set correspondingly.

Indicator as New Options Extension to an IP Extension of an IP Header

On the basis of existing IP options mechanism (e.g. as explained abovewith respect to strict and loose routing), an IP packet header isextended by a so-called “multiple anycasting” options field containingan indicator as specified above. In case a packet is destinated for ananycast address, the respective header including anycasting addressinformation, anycast capable routers only analyses the extension, namelythe “multiple anycasting” option field. Modifications of the extensioncan be performed, for example, to update the “multiple anycasting”option field to be in line with the number of anycast members still tobe contacted/allocated in view of already contacted/allocated anycastmembers.

As set forth above, data communications are differently performed forintra-domain and inter-domain communications. As a result, multipleanycasting will work different for intra-domain cases employing interiorgateway routing protocols such as OSPF and inter-domain cases employingexterior gateway protocols such as BGP. In the following inter-domainmultiple hosts anycasting and intra-domain multiple hosts anycastingwill be described with reference to FIG. 5 illustrating an exemplarytopology and routing scenario wherein a host H in a domain Ccommunicates an anycast packet within a network comprising domains A, Band C and exterior gateway routers R1, R2, R3 and R4. For this exampleit is assumed that the anycast group addressed by the host H comprisesthe anycast members A1, A2 and A3 in domain A and anycast members A4 andA5 in domain B. In particular, host H communicates an anycast packetincluding an indicator specifying a number of three anycast members tobe contacted as receivers, i.e. nearest anycast members.

Inter-domain Multiple Hosts Anycasting

Inter-domain multiple hosts anycasting is indicated by steps 1 and 3shown in FIG. 5. On the basis of an employed exterior gateway routingprotocol, the first exterior gateway router with respect to host H indomain C, i.e. router R4, will check its routing tables in order todetermine anycast members of the anycast group as specified by theanycast packet of host H and, in particular, all domains including therespective anycast members. The corresponding routers for determineddomains, i.e. routers R1 and R3 to domains A and B, will be added to theIP packet header of the anycast packet from host H by utilizing strictor loose routing options. Depending on the metrics used for the employedexterior gateway routing protocol and/or the smallest distance metricrequested by the sender, i.e. host H, (e.g. defined in the TOS field),the ordering of the routers will be based on distances between domainsincluding anycast members and anycast members, respectively, and host H.According to FIG. 5, the anycast packet of host H will be firstcommunicated to router R3. In case the number of anycast memberscontacted/allocated in a domain receiving the anycast packet from routerR3 is less than the number of nearest anycast members to be contacted asdefined by host H, the anycast packet will be communicated to router R1.The anycast packet and, in particular, the header thereof transmitted torouter R1 includes an indicator specifying the number of anycast memberswhich are still to be contacted/allocated. With reference to FIG. 5, theindicator communicated to router R1 indicates that one anycast member isto be contacted/allocated assuming that anycast members R4 and R5 ofdomain B are selected as nearest anycast members.

As specified by the strict or loose routing options of the IP packetheader, the specified number of anycast members may be contactedsequentially. Assuming the specified number being rather low, e.g. 2 to5 anycast members, this procedure will not effect the network serviceperformance received by requesting host, here host H. In case a highnumber of anycast member should be allowed to be specified by asender/requesting host, various known data communication methods forenhancement of data transmissions in a network can be utilized forcontacting anycast members.

Intra-domain Multiple Hosts Anycasting

Intra-domain multiple hosts anycasting will be explained with referenceto steps 2 and 4 of FIG. 5. At reception of an anycast packet in anborder router for a domain, the border router will forward the anycastpacket to all next hops, in particular, to the next anycast members ofthe respective domain. This can be accomplished since, as explainedabove, interior gateway routing protocols provide a separate entry inthe routing table for each anycast member. For forwarding the anycastpacket, the border router may multiply the anycast packet andsimultaneously communicate the multiplied anycast packets to multipledestinations, i.e. next hops/anycast members. According to the number ofanycast members and, in particular, of anycast memberscontacted/allocated in the corresponding domain, the number specified bythe indicator in the header of the anycast packet is decreased. In casethe indicator is 0 after an intra-domain multiple host anycasting, nodata communications are required to contact/allocate further anycastmembers, e.g. of a different domain, since the number of nearest anycastmembers as desired by a requesting host is provided. Otherwise, theborder router will forward the anycast packet including the decreasedindicator to the next exterior gateway router for inter-domain multiplehosts anycasting as specified above.

According to FIG. 5, a border router of domain B receives the anycastpacket from exterior gateway router R3 and will forward the anycastpacket to the anycast members A4 and A5. For example, router R3 mayserve as border router for domain B by combining exterior and interiorgateway functions. In step 2 the border router of domain B contacts allanycast members in domain B such that they are specified for nearestanycast members for data communications with host H. As a result, stillone further anycast member has to be contacted/allocated, whereby theanycast packet communicated from the border router of domain B via theexterior gateway router R3 to the exterior gateway router R1 includes acorrespondingly decreased indicator, i.e. an indicator having the valueof 1. A border router of domain A forwards the received anycast packetwithin domain A as specified above for contacting/allocating an anycastmember. Comparable to router R3, router R1 combining exterior andinterior gateway capabilities may be operated as border router fordomain A.

The routing within a domain may be performed in a manner comparable asthe above described inter-domain routing in case hierarchies are usedfor the respective domain. Then, all levels of the same hierarchy in thedomain can be contemplated as inter-domain cases, wherein levels of alower hierarchy can be seen as intra-domain cases.

Skipping the Number of Anycast Members Specified in a First MultipleHosts Anycasting for Performing a Second Multiple Hosts Anycasting

After having performed a first multiple hosts anycasting, it is possiblethat the respective host performs a second multiple hosts anycastingwherein the number of anycast members specified in the first multiplehosts anycasting should not be contacted/allocated (again). For thatpurpose it is contemplated as an ad-on to the above described multiplehosts anycast routing, but also to the existing anycast specification asknown in the state of the art, that a sending/requesting host canspecify the number of first anycast members (i.e. anycast memberscontacted/allocated in a first anycasting) that should be notcontacted/allocated (again) in a second (multiple hosts) anycasting.

After a previous attempt did not result in a successful anycast membersearch there may be no need to contact/allocate the same anycast membersonce more if the number of anycast members is to be extended in asubsequent attempt. In that case, the sending/requesting host couldspecify in a new optional IP header extension the number of firstanycast members that should not be contacted/allocated in the subsequentattempt. Such an indication can be provided by a second indicator(counter) which is additionally checked, e.g. by routers, whileperforming the second (multiple hosts) anycast routing.

In case a network domain includes anycast members, the additionalindicator (counter) is first checked. If the second indicator indicatesa number greater than 0, the second indicator is to be decreased untilit reaches 0. For each decrease of the second indicator, the firstmultiple hosts indicator (counter), as described above, is decreased aswell, but without contacting/allocating an anycast group member. In thisway, it is ensured that the number of anycast members contaced/allocatedin the first, preceding (multiple hosts) anycasting is not contactedagain in the second attempt. As set forth above, this optional additionmay already be implemented in the single hosts anycasting as known inthe state of the art.

1. A method for multiple hosts anycast routing in a network, the methodcomprising the steps of: communicating sender data from a senderaddressed to an anycast group including anycast members, said senderdata including an anycast address for addressing said anycast group anda first indicator specifying a first number of said anycast memberswithin said anycast group for data communications with said sender, anddetermining a plurality of nearest ones of said anycast members withinsaid anycast group as hosts for said data communications with saidsender, wherein said plurality of nearest ones of said anycast memberscorresponds to the number specified in said first indicator, wherebysaid sender can communicate with said plurality of nearest ones of saidanycast members without separately addressing each one thereof.
 2. Themethod according to claim 1, further comprising the steps of:communicating further sender data from said sender addressed to saidanycast group including said anycast members, said further sender dataincluding a second indicator specifying a second number of said anycastmembers within said anycast group, said second number being indicativeof previously-determined nearest ones of said anycast members withinsaid anycast group, and determining said nearest ones of said anycastmembers for communication with said sender as a function of said firstand said second indicators.
 3. The method according to claim 2, whereinsaid number of nearest anycast members to be determined is defined bythe difference between said first number and said second number.
 4. Themethod according to claim 2, wherein said first and/or said secondindicator is included in said anycast address.
 5. The method accordingto claim 2, wherein said anycast address is extended by said firstand/or said second indicator.
 6. The method according to claim 2,wherein an anycast address prefix is included in said anycast address,said anycast address prefix identifying a topological region in saidnetwork confining said anycast group, said anycast address prefixincluding said first and/or said second indicator or being extended bysaid first and/or said second indicator.
 7. The method according toclaim 2, wherein said anycast address and said first and/or said secondindicator are communicated by communicating an Internet Protocol datapacket header including said anycast address.
 8. The method according toclaim 7, wherein said first and/or said second indicator is included insaid Internet Protocol data packet header as a part or an extension ofsaid anycast address, or said Internet Protocol data packet header isextended by said first and/or said second indicator by means of anoptions field for Internet Protocol data packet headers.
 9. The methodaccording to claim 2, wherein said anycast address and said first and/orsaid second indicator are communicated in said network between domainsthereof via routers to network domains having at least one of saidanycast members.
 10. The method according to claim 9, wherein anordering of said routers is defined such that said anycast address andsaid first and/or said second indicator are communicated to the nearestnetwork domains, the nearest network domains being specified accordingto a measure of distance of a routing protocol for said network and/oraccording to a measure of distance specified by said sender.
 11. Themethod according to claim 2, wherein said first indicator is modifiedaccording to a number of already determined nearest anycast members suchthat said modified indicator specifies a number of said anycast membersstill to be determined as receivers.
 12. The method according to claim11, wherein said first indicator is decreased by the value of saidsecond indicator.
 13. The method according to claim 2, wherein saidsecond indicator is decreased to a value corresponding to a number of 0and said first indicator is decreased correspondingly to the decrease ofthe second indicator and said determining of said number of nearest onesof said anycast members within said anycast group is performed independence of said decreased first indicator if said second indicatorhas said value corresponding to said number of
 0. 14. The methodaccording to claim 2, wherein said first and said second indicator arecommunicated together.
 15. The method according to claim 1, wherein saidnumber of nearest anycast members is determined by utilizing a measureof distance of a routing protocol for said network and/or determined byutilizing a measure of distance specified by said sender.
 16. The methodaccording to claim 1, wherein said number of anycast members iscontacted sequentially.
 17. The method according to claim 1, whereinsaid first number specified by said first indicator is decreasedcorresponding to a number of said anycast members in a network domain towhich said anycast address and said first indicator are communicated.18. The method according to claim 1 wherein said communicating isperformed according to a strict routing mechanism or a loose routingmechanism as defined for Internet Protocol.
 19. A method for multiplehosts anycast routing in an anycast capable network, the methodcomprising the steps of: associating a first indicator with an anycastaddress identifying an anycast group having anycast members in saidnetwork, said first indicator specifying a first number of said anycastmembers within said anycast group, communicating at least said anycastaddress and said first indicator in said network to said anycast group,and determining a plurality of nearest ones of said anycast memberswithin said anycast group as hosts for data communications in saidnetwork, said plurality of nearest ones of said anycast members beingdefined by said first indicator, whereby said sender can communicatewith said plurality of nearest ones of said anycast members withoutseparately addressing each one thereof.
 20. The method according toclaim 19, further comprising the steps of: associating a secondindicator with said anycast address, said second indicator specifying asecond number of said anycast members within said anycast group, saidsecond number being indicative of previously-determined nearest ones ofsaid anycast members within said anycast group, communicating at leastsaid anycast address, said first indicator and said second indicator insaid network to said anycast group; and, determining said nearest onesof said anycast members for communication with said sender as a functionof said first and said second indicators.
 21. The method according toclaim 20, wherein said number of nearest anycast members to bedetermined is defined by the difference between said first number andsaid second number.
 22. The method according to claim 20, wherein saidfirst and/or said second indicator is included in said anycast address.23. The method according to claim 20, wherein said anycast address isextended by said first and/or said second indicator.
 24. The methodaccording to claim 20, wherein an anycast address prefix is included insaid anycast address, said anycast address prefix identifying atopological region in said network confining said anycast group, saidanycast address prefix including said first and/or said second indicatoror being extended by said first and/or said second indicator.
 25. Themethod according to claim 20, wherein said first indicator is modifiedaccording to a number of already determined nearest anycast members suchthat said modified indicator specifies a number of said anycast membersstill to be determined as receivers.
 26. The method according to claim25, wherein said first indicator is decreased by the value of saidsecond indicator.
 27. The method according to claim 19, wherein saidnumber of nearest anycast members is determined by utilizing a measureof distance of a routing protocol for said network and/or determined byutilizing a measure of distance specified by said sender.
 28. A systemfor multiple hosts anycast routing, comprising: a network a sender, anda group of receivers identified by a common anycast address, whereindata communications between said sender and a number of nearest ones ofsaid receivers are established based on a first indicator provided bysaid sender specifying said number of receivers, whereby said sender cancommunicate with a plurality of nearest ones of said anycast memberswithout separately addressing each one thereof.
 29. The system accordingto claim 28, wherein said data communications between said sender andsaid number of nearest of said receivers are further established basedon a second indicator provided by said sender specifying a number ofreceivers previously-determined for said data communications with saidsender.